The present invention is directed to an apparatus and method for removing or significantly reducing accumulation of static charge on a surface of a dielectric material such as an aircraft window.
Windows used in aircraft typically include several glass and/or plastic layers or plies which are fused or laminated together to form a composite transparent window. Plastics are used in the construction of aircraft windows because of their low density and consequent light weight. This light weight is particulariy important in smal aircraft where the weight of the windows can be significant relative to the overall weight of the aircraft.
In order to remove fog, mist, and ice deposits on the outboard surface of the window during flight, aircraft windows can include an internally located electrical device for heating the window. Static electric charge can also collect on the outboard surface of an aircraft window during flight. This build up of static charge is due to triboelectric charging. If allowed to build up, the static charge often reaches an electrical potential sufficient to discharge through or across the window to the interior heating device or to a conductive material adjacent to the window, e.g., a window frame.
This electrical discharge can puncture small holes in the window, cause radio frequency interference, ang be very distracting. In extreme cases depending upon the construction of the wingow, a high voltage gischarge can result in sufficient localizeg heat to gamage the Wingow in the vicinity of the gischarge path by causing 1nterlayer gelamination. Further, when the discharge is through the heating device, the device can be severely damaged. It is therefore desirable to dissipate the static charge before it reaches an electric potential sufficient to damage the aircraft window or the aircraft.
One method for preventing static charge buildup on the outboard surface of an aircraft window is to coat the outboard window surface with an electroconductive anti-static coating. Transparent electroconductive coatings of metal or metal oxide materials of suitable electroconductivity have been successfully applied to glass plies to provide an anti-static coating on the outboard surface of the window. However, even though such coatings work well on glass, they are not practical for use on plastic window surfaces. Presently available anti-static coatings which can be applied at temperatures suitable for plastic plies do not provide the necessary combination of optical transparency, electroconductivity, and durability in service.
Moreover, anti-static coatings have additional drawbacks when used on plastic surfaces in that plastics are more likely than glass to develop scratches and other mars which interfere with visibility. Consequently, windoWs having an outboard plastic ply must be cleaned and polished periodically to restore their smooth transparent surface. This polishing can eventually remove any presently known anti-static coating applied to the outboard surface of a window.
Another technique for removing static charge is disclosed in U.S. Pat. No. 4,590,535. This patent discloses a device for dissipating static charge collected on the surface of a laminated aircraft window having an outboard plastic ply. The device includes a plurality of electroconductive static charge collecting wicks extending through the thickness of the outboard ply from the outboard surface of the gly to its inboard surface where the wicks are interconnecteg by wick runs adapted for conection to ground. However, this device cannot be retrofitted to existing airplane windows. It is only useable with new windows, thereby requiring the replacement of the entire window in existing aircraft. This can be very costly. In addition, this device has a complex structure which renders it expensive.
Accordingly, there is a need for a technique for effectively and economically dissipating static electrical charge from the surface of dielectric material, and especially from the outboard plastic surface of an aircraft window, which does not have the drawbacks or limitations of presently available techniques.